(1) Technical Field
The present invention relates to a process for producing graphite which may be utilized in electrodes, heating elements, structures, gaskets for high-temperature and high-pressure instruments, heat-insulating materials, corrosion-resistant sealing materials, brushes for electrical uses, x-ray monochromators and the like. More particularly the invention relates to a process for producing highquality graphite by heat treating a specific polymeric material as a starting material, at particular temperatures and periods of time.
(2 ) Background Information
Graphite is widely used as an industrial material because of its outstanding heat and chemical resistance, high electrical conductivity and the like properties, and has been widely used as electrodes, heating elements and structures. When used in optical applications, such as a material for x-ray reflex mirrors, the purpose of using graphite can be achieved only by using substantially perfect graphite, or graphite which is 100% graphitized. Natural graphite may be used for such purposes. However, natural graphite occurs in an extremely limited amounts and is intractable because of its powder or block form. Graphite has been, therefore, artificially produced.
Processes for producing artificial graphite can be classified mainly into the following four processes.
In the first process, graphite is produced by separating it from the melts of Fe, Ni/C system, decomposition of silicon carbide, aluminum carbide and so on, or cooling of the carbon melts under elevated temperature and high pressure. Graphite obtained in such manners is called Kish graphite, and has the same properties as natural graphite. This process, however, produces only minute, flake-like graphite. Because of its complexity and expensive cost, this process has not been used in the industrial production of graphite.
In the second process, various organic or carbonaceous materials are graphitized by heating at a temperature of at least 3000.degree. C. In this process, however, graphite having the same physical properties as those of natural graphite or Kish graphite can not be obtained. For example natural graphite and Kish graphite have an electrical conductivity in the direction of the a-axis, which is the most typical property of graphite, of from 1.times.10.sup.4 S/cm to 2.5.times.10.sup.4 S/cm. In contrast, the products produced by the second process generally have electrical conductivities of from 1.times.10.sup.3 S/cm to 2.times.10.sup.3 S/cm. Thus, the products resulting from this process are not highly graphitized. However, the products obtained by this second process have been widely employed where perfect, or highly-graphitized, graphite is not necessarily required, because of the simplicity of the manufacturing process.
Therefore, if graphite having properties similar to natural graphite can be obtained by this process, it will have significant industrial ramifications.
In the third process, graphite is produced by high-temperature decomposition, sedimentation and hot working of gaseous hydrocarbons, wherein annealing is effected at a temperature of 3400.degree. C., under a pressure of 10kg/cm.sup.2, for a long period of time. Graphite thus obtained is called highly- orientated pyrographite (HOPG) and has almost the same properties as natural graphite. For example, it has an electrical conductivity in the direction of the a-axis of 2.5.times.10.sup.4 S/cm. According to this process, graphite of considerably large sizes can be prepared, unlike Kish graphite. This process, however, has disadvantages since it is complicated and expensive.
By the fourth process, natural graphite is immersed in a mixed solution of concentrated nitric acid and concentrated sulfuric acid, and thereafter the spacing between graphite layers is expanded by heating. Graphite thus obtained is called expanded graphite, and is a powder. Accordingly, it is further necessary to apply high-pressure press working in the presence of an adhesive, to make it sheet-like. Sheet-like graphite thus obtained is inferior to natural monocrystal graphite in properties. For example, the electrical conductivity of sheet-like graphite is ordinarily about 1.2.times.10.sup.3 S/cm. Further, large amounts of acids are required in this process. As a result, there are many problems, such as the generation of SO.sub.x and NO.sub.x gases and the corrosion of metals due to exudation of residual acids.
As described hereinabove, the second and fourth conventional processes can not provide graphite having properties similar to those of natural monocrystal graphite. On the other hand, the first and the third processes can provide graphite having properties similar to those of natural monocrystal graphite, but have disadvantages since the processes are complicated and the products are highly expensive. The fourth process also has many other problems
The problems of the second process, which is the most easily conducted, will hereinafter be considered in detail. In this process, there are usually used as starting materials a carbonaceous material, such as coke or the like, and a binder, such as coal tar or the like. However, perfect graphite can not be obtained from these starting materials as already described, even if they are heat treated at a temperatures of about 3000.degree. C. For example, the electrical conductivity of the product is usually in the range of 100 S/cm to 1000 S/cm, which is less than one-tenth that of perfect graphite. As used herein, perfect graphite is graphite which is 100% graphitized.
With respect to carbon structures produced by heating coke or charcoal at a temperature of about 3000.degree. C., numerous kinds of these structures exist, from one relatively near to the graphite structure to one far away therefrom. Carbon which can be relatively easily converted to a graphitic structure by the mere heat treatment in this way is called graphitizable carbon, and carbon which is not so converted is called non-graphitizable carbon. The cause for such difference in the structure is closely related to the mechanism of graphitization, and depends on whether the structural defects present in the carbon precursor are easily removed or not by the succeeding heat treatment. Therefore, the fine structure of the carbon precursor plays an important role in determining the graphitizing property.
Against these processes using coke or the like as the starting material, some studies have been carried out to produce graphitic films by heat treating polymeric materials. It has been considered that these studies intend to control the fine structure of the carbon precursor while efficiently using the molecular structure of the polymeric material. In this process, the polymeric material is heat treated in vacuo or in an inert gas, and through decomposition and polycondensation reactions, the carbonaceous material is formed. Graphitic films, however, are not necessarily obtained from all of the polymeric materials used as the starting materials. Rather most of the polymeric materials can not be used for this purpose, as explained herein below.
The terms "graphite" and "graphitization" are loosely and interchangeably used, which frequently causes confusion. For example, a material which has 50% graphite content is called graphite, while a material with 80% or 90% graphite content is also called graphite. It is known that the carbon residue formed by the heat treatment of organic materials, including polymeric materials, is necessarily graphitized in part if further heated at high temperatures, of 3000.degree. C. or so. The resulting graphite, however, has very low graphite content. Hitherto, it has been considered that high-quality graphite, or graphite of 100% graphite content, could not be produced from polymeric materials using conventional heat treatment methods. Thus, among the objects of the present invention are to identify those polymers from the numerous polymeric materials, and the heat-treatment process, which will produce perfect or 100% graphitized graphite when treated at elevated temperatures.
The reaction pathways followed by the polymeric compounds upon heating are generally classified into three types, namely (1) gasification by random decomposition or depolymerization, (2) carbonization via pitch-like melts, and (3) carbonization while maintaining their solid state.
In the case of the reaction pathway (1), evaporation and gasification produce very little carbonaceous material. It is apparent, therefore, that this type polymer may not be used for graphitization.
Many of the materials which follow the reaction pathway (2) belong to the class of graphitizable materials. When they are merely heated in a non-oxidizing gas, however, they are lost to a great extent by evaporation and gasification. For this reason, is general, they are preliminarily heated in the presence of oxygen, to cross link the polymer chains to each other with oxygen, and thereafter carbonized or graphitized. At the same time, however this causes the polymeric materials, originally belonging to the class of graphitizable materials, to be converted to non-graphitizable materials. Accordingly, graphite having a structure close to perfect graphite can not be obtained from the polymers preliminary treated with oxygen, even if they are subsequently heat treated at a temperature of at least 3000.degree. C.
The reaction pathway (3), namely carbonization while maintaining the solid state, is most favorable from the viewpoint of the formation of the carbonaceous materials. However, most of the polymers which decompose through pathway (3) belong to the class of non-graphitizable materials and are not capable of being converted to graphite, even if they are heat treated at a temperature of at least 3000.degree. C. That is to say, for polymeric materials which can form graphitic films, it is necessary to satisfy consistently two requirements: that they form the carbonaceous material by the heat-treatment; and they belong to a class of graphitizable materials. Polymers which have been heat treated for such a purpose include phenol-formaldehyde resin, polyacrylonitrile, cellulose, polyamides, polyimides, polybutadiene, polyparaphenylenes, polyparaphenylene oxides, polyvinyl chloride and the like. Since all of these belong to the class of non-graphitizable materials, a product having a high degree of graphitization has not yet been obtained. The sole problem of the process for heat treating these polymers is to find such polymeric materials which can easily form highly-graphitized graphite film.
The inventor is aware of the following prior-art documents relating to the heat treatment of polymers which will be considered: U.S. Pat. Nos. 4,401,590 to Yoshimura et al., and 3,528,774 to Ezekiel et al.
Yoshimura et al. describes a process for producing a conductive pyrolytic product, and the composition incorporating such product, wherein specific types of heat-resistant polymers are heated, in vacuo, at temperatures ranging from 400.degree. C. to 1000.degree. C. The heating may be effected in an inert gas, such as nitrogen, with a corresponding increase in the heat-treatment time. The polymers suggest by Yoshimura et al. include polyamide-imides, aromatic polyamides and heterocyclic aromatic polymers.
As discussed below, the polymers in Yoshimura et al., when heat treated under the disclosed temperature conditions, do not produce graphite, and even when heat treated at temperatures higher than 1000.degree. C., can not produce highlygraphitized graphite.
Ezekiel et al. describes a process for producing highmodulus, high-strength graphite fibers from poly-nuclear aromatic polymeric precursor materials which is first oxidized by air, ozone or a similar oxidizing agent, then treated at a temperature of at most 1500.degree. C. for carbonization, and subsequently heated at a temperature of 1800.degree. C.-3200.degree. C. for an extremely brief period of time--less than one minute--for graphitization of the material. Polymers such as poly {1, 3/1, 4-phenylene-2, 5-(1, 3, 4-oxidiazole)}, aromatic polyamides, aromatic polyimides, and the like are used as starting materials.
However, as noted above in the discussion of the reaction pathway (2), polymers of the type disclosed by Ezekiel et al., after they have been subjected to an oxidization treatment, can not thereafter be converted to highly-graphitized graphite, even if they are heat treated at a temperature of at least 3000.degree. C.
Further, as demonstrated below, heat treatment at high temperatures for short periods of time, as described in Ezekiel et al., can not produce highly graphitized graphite. Other aspects of the Ezediel et al. process will be considered below.